1. Field of the Invention
This invention generally relates to techniques for removal of volatile contaminants from substrate surfaces before the substrates are placed into a vacuum process chamber. In particular, the invention relates to techniques for cleaning semiconductor wafers, as well as other articles used in manufacturing of semiconductor devices using a charged particle beam, such as an electron beam.
2. Description of the Related Art
Modern microelectronic devices such as microprocessors and memory chips are manufactured by creating multiple layers of various materials on the top surface of a silicon wafer. For instance, semiconductor device fabrication can involve creating a silicon oxide layer on the surface of the wafer, and creating interconnecting wires by depositing a metal layer having a predefined microscopic pattern on the top surface of the wafer.
If the surface of the semiconductor wafer has various contaminants, such as volatiles, water, or oxygen molecules, the semiconductor devices manufactured using such a wafer may be defective. Contamination of the wafer surfaces is a major cause of reduced process yields in device manufacture. Therefore, as will be appreciated by those of working skill in the art, during manufacture of semiconductor devices it is desirable to keep the surfaces of the semiconductor wafers free of any contaminants.
A major cause of the aforementioned wafer surface contamination is electromagnetic interaction between the atoms and molecules of the contaminant and the atoms of the material (e.g. silicon, or silicon oxide) forming the top atomic layers of the wafer. Such electromagnetic interaction results in an attractive force between the contaminant atoms/molecules and the wafer atoms/molecules, thus keeping the contaminants attached to the surface of the wafer.
Various contaminants come from different sources. Some of the most common contaminant species include fluoride (F−) ions, which originate generally in the vacuum system. Hydrogen (H+) contamination comes from the residual vacuum as well as from the H+ contamination of the wafer during its preparation. This type of contamination can be hard to remove. Other contaminants include O+, H2O, and various hydrocarbons, which come primarily from the ambient atmosphere.
One conventional way of removing the contaminants from surfaces of substrates is heat treatment. According to one such method, the surface of the substrate is illuminated with a powerful beam of electromagnetic radiation, from a lamp or similar device. The irradiation of the substrate surface causes heating of the surface. The heating, in turn, increases the thermal energy of the surface and the contaminants attached thereto, whereby the contaminants leave the surface when their thermal energy exceeds the size of the potential barrier created by the aforementioned attractive force. Typically, for effective removal of the contaminants, the surface of the substrate needs to be heated to temperatures of 200–300° C. or higher, depending on the nature of the contaminants and the chemical composition of the substrate's surface layer.
As also will be appreciated by those of working skill in the art, any unnecessary heating of the surface of a semiconductor wafer is undesirable. In particular, heating a silicon wafer may cause permanent damage to the crystalline structure thereof, change profiles of dopant levels, for example, or release contaminants embedded in the body of the wafer and its internal structures.
Accordingly, there is a need for, and it would be advantageous to have an improved substrate surface cleaning method and apparatus, which would provide efficient surface cleaning without substantial heating of the substrate's surface.